When an inductor is used as part of an AC filter in a DC link, the total DC current flows through the inductor. As a result of the current flow, the energy stored in the inductor is ½ LI2, where L is the inductance of the inductor and I is the current. A load connected uses some of this energy and the remainder is stored in the inductor. Because of this, the inductor has to be sized to store the energy consumed by the load plus the stored energy.
Significant DC current in an inductor leads to magnetic saturation of its magnetically permeable core. Generally, an air gap has to be inserted somewhere in the magnetic path of the core in order to avoid such DC induced magnetic saturation. The air gap has the effect of increasing the length of the magnetic path. As the magnetic path length is increased, the magnetic, or H, field decreases. This places the magnetic operating point of the inductor in the linear region of its hysteresis, or B-H, loop where the permeability of the core is relatively large.
Even though the core permeability is large, the air gap causes the effective permeability to be less than the core permeability. Since the inductance is proportional to the effective permeability and inversely proportional to the magnetic path length, the insertion of the air gap decreases the inductance of the inductor. Since the air gap is necessary to avoid magnetic saturation, the number of turns of the inductor has to be increased, the area of the core has to be increased, or both in order to make up for the inductance loss caused by insertion of the air gap. It is usually preferable to increase the core area, since the addition of turns also increases the H field, and that may require an increase in the air gap. In any case, the presence of DC current in an inductor requires that the inductor be sized larger than if no DC current were present.
Another way to reduce the increased H field due to DC inductor current is to insert another H field through the magnetic path of the inductor that has an opposite orientation. The net H field is thus reduced and the magnetic operating point of the inductor may be maintained in the linear region of the hysteresis loop without a lengthy air gap. In this way, the air gap may be shortened or eliminated and the effective permeability of the inductor shall be greater. Since the effective permeability is larger, the inductor core size may be reduced compared to a similar inductor without the inserted H field of reverse orientation.
In the past, such an oppositely oriented H field has been introduced with a permanent magnet so positioned relative to the inductor to oppose the inductor DC current induced H field. This method can work satisfactorily, but it has two serious drawbacks. When the permanent magnet is made, the H field of the magnet has to be controlled to a prescribed level that satisfactorily cancels the inductor DC current induced H field when mounted proximate the inductor. Another drawback is that the H field of the permanent magnet is static, and therefore cannot be controlled after the permanent magnet is mounted proximate the inductor. Thus, if the inductor DC current is variable, the H field due to the permanent magnet may dominate when the inductor DC current is low and the H field due to the inductor DC current may dominate when the inductor DC current is very high. The third drawback is that a permanent magnet has a low permeability and therefore it introduces an equivalent air gap into the magnetic path of the inductor.